Convertible furniture
A piece that changes function — sofa-cum-bed, wall bed, fold-down desk, nesting tables. The hardware of layering. Judged by three tests: one-person-one-minute, no-compromise in either mode, and a home for the props it displaces.
The hardware of layering
Most convertibles are bought for the dream and abandoned in a month — the sofa-bed that's a bad sofa and a worse bed. The more often a piece switches, the lower its friction must be; for daily layers, spend on the mechanism.